1. Field of the Invention
This invention relates to a communication system that uses mobile IP, and especially relates to a method for selecting a communication IP address in a mobile node that moved from its home network to a foreign network.
2. Description of the Related Art
In the Internet, it is common that when a mobile node moved among networks, the mobile node newly acquires an IP address in a network to which it moved (hereinafter sometimes referred to as “visited”) and performs communication with it.
On the other hand, IETF is now standardizing a method whereby a mobile node performs communication in a visited network to which it moved (hereinafter a network being referred to as a “foreign network”) without altering the IP address of the mobile node by using the IP address that has been assigned to the mobile node (hereinafter referred to as a “home address”) in a network to which the mobile node is usually attached (hereinafter referred to as a “home network”).
In “Mobility Support in IPv6” (David B. Johnson, draft-ietf-mobileip-ipv6-20.txt, Jan/2003) that is being examined in IETF (hereinafter referred to as a “mobile IP”), a terminal that wishes to communicate with a mobile node sets a destination IP address as the mobile node's home address and sends an IP packet. The IP packet that was routed to the mobile node's home address is captured by the mobile node's home agent and is transferred after being encapsulated to a foreign network visited by the mobile node.
In order to encapsulate and forward the IP packet that was received, the home agent receives a binding update from the mobile node and manages association between the mobile node's home address and the IP address that the mobile node acquired in the foreign network (hereinafter referred to as a “care-of address”). This procedure makes it possible for the mobile node to receive the IP packet addressed to its home address in the foreign network. When communication starts between the mobile node and the terminal, the binding update will be sent to the terminal of a partner of communication from the mobile node; therefore, it is also possible for the mobile node and the terminal to communicate directly between each other henceforth not via the home agent but by using a routing header that is an extension header of IPv6. Moreover, if a home agent exists on a first foreign network that is a network visited by the mobile node, after the mobile node has moved to a second foreign network, it is also possible to forward a packet addressed to the mobile node to the mobile node residing in the second foreign network by sending the binding update to the first foreign network's home agent.
[Non-Patent Literature 1]
David D. Johnson et al. “Mobility Support in IPv6 draft-ietf-mobileip-ipv6-20.txt,” obtained online from IETF on Jan. 20, 2003, also available from the Internet <http://www.ietf.org/internet-drafts/draft-ietf-mobileip-ipv6-20.txt> [searched on Feb. 5, 2003]
With the technology described above, the mobile node will be able to communicate with the terminal using its home address even when it has moved among networks. Moreover, the mobile node in a foreign network can select either its home address or the care-of address that was acquired in the foreign network as a communication address.
For example, if the communication address is set to the mobile node's home address, the mobile node can receive a packet surely even when it moves from one network to another. However, in this case, since the IP packets are encapsulated and forwarded by the home agent to reach the mobile node, an overhead of communication is created. After the start of the communication, it is possible to eliminate the overhead of communication by sending the binding update to a partner of the communication and hence setting up route optimization. However, it is impossible to set up the route optimization from the time of start of the communication.
On the other hand, in the case where the communication address is set to a care-of address, since the mobile node can directly communicate with the partner of communication, the overhead of communication can be eliminated. However, when the mobile node has left the first foreign network and moved to the second foreign network, IP packets addressed to the mobile node that have reached the first foreign network may not be forwarded to the second foreign network, and so there is the possibility that the mobile node may not be able to maintain a session with the partner of communication.